1. Field of the Invention
The invention relates generally to a method of manufacturing a transistor in a semiconductor device. More particularly, the present invention relates to a method of manufacturing a transistor in a semiconductor device by which a semiconductor substrate is isolated by an oxide layer with only a source, a drain and a channel region necessary for driving a transistor being left, thus obviating the current components due to parasitic factors to improve the punch-through characteristic.
2. Description of the Prior Art
A method of manufacturing a conventional MOS transistor will be explained by reference to FIG. 1. A device isolation film (not shown) for isolating a cell region and a field region is formed on a semiconductor substrate 11. After a well 12 is formed on the semiconductor substrate 11, an ion implant process for adjusting the threshold voltage is performed. Then, a gate oxide film 13 is formed on the semiconductor substrate 11. Thereafter, a polysilicon layer, a polysilicon layer/a metal silicide layer or a metal layer, etc. is deposited on the gate oxide film 13 and is then patterned to form a gate electrode 14. Next, after a spacer insulating film 15 is formed on the sidewall of the gate electrode 14, a source/drain junction 16 is formed on the semiconductor substrate 11 by source/drain ion implant process.
In the transistor manufactured by the above method, if the voltage is applied to the drain and the gate is turned on, current will flows from the drain to the source. On the other hand, if the gate is turned off, the current from the drain to the source will be blocked. However, if the voltage applied to the drain is too great, unwanted current will flow into the well. This current is called a punch-through current. The punch-through current is generated when carriers are applied to a depletion layer and are then attracted by a bias of the depletion layer. This phenomenon becomes still severe as the length of the gate becomes narrower and will limit to manufacturing higher devices.
It is therefore an object of the present invention to provide a method of manufacturing a transistor in a semiconductor device capable of improving reliability of the device, by removing a well being the cause of generation of the punch-through so that the punch-through current does not generate.
In order to accomplish the above object, a method of manufacturing a transistor in a semiconductor device according to the present invention is characterized in that it comprises the steps of providing a semiconductor substrate and etching one part of the semiconductor substrate to form a first trench at a position where a transistor will be formed; etching the other part of the semiconductor substrate to form two trenches at a position where a source junction and a drain junction will be formed; implanting oxygen ions into the entire portions of the semiconductor substrate including the first and second trenches and then performing a thermal process to form a buried oxide layer within the semiconductor substrate; isolating, by the buried oxide layer, the semiconductor substrate of the portion where a source/drain junction will be formed, and of the portion where a channel region will be formed, by polishing process; and performing ion implant process for controlling the threshold voltage to form a channel region and then forming a gate oxide film, a gate electrode and a source/drain junction.